N Biogeochemistry

Calculating a Nitrogen Mass Balance for California

Mass Balance flows - figure 1

California's Nitrogen Flows (click to enlarge)

Nitrogen is our atmosphere's most prominent element, and estimated at being among the most abundant elements in the solar system.  In its inert form (N2), nitrogen does little for plants and animals on earth.  But when nitrogen is incorporated into different compounds, it becomes a powerful tool for both human use and for the earth itself.  Nitrogen is key for the development of proteins, amino acids and nucleic acids.  Plants grow greener and animals grow stronger with nitrogen in their systems. 

Nitrogen becomes accessible (reactive nitrogen) in the environment through the process of fixation--some of which happens through lightening strikes, but is mostly performed by bacteria found in the roots of leguminous plants.  But the ability for plants to fix nitrogen, and the abundance of nitrogen-fixing plants in an area can be a major limiting factor in plant growth.

The human ability to mimic the fixation process and convert N2 into biologically useable compounds has enabled us to dramatically increase food production for a growing planet.  Synthetic nitrogen fixation, known as the Haber-Bosch process, creates reactive nitrogen under high temperatures and pressure, combining N2 with hydrogen to form ammonia (NH3), used to make fertilizer and explosives. 

With the increase in nitrogen availability comes an array of complexity.  Nitrogen travels quickly through our ecosystems, changes forms and affects the environment in a variety of ways.  As we add more and more nitrogen to the environment, more of it is transferred between systems, and more of it accumulates in our land, air and water.  This mobilization of nitrogen is connected with increased N loading to aquatic ecosystems, emissions of nitrous oxide (a greenhouse gas), and the ecosystem and human-health effects that accompany them.  

nitrogen compounds table

Major nitrogen compounds

In some cases, a nitrogen flow is an inherent component of an ecosystem service.  When we harvest crops for food, nitrogen is harvested with those crops and begins to travel through the environment and our bodies.  Other flows can be considered side effects of nitrogen use, and are linked with impairing ecosystems and harming human health.  Reactive nitrogen finds its way into the environment through fossil fuel combustion, converting inert nitrogen into nitrogen oxides (N2O, NO2, NO), which are potent greenhouse gases and contributors to acid rain and harmful particulate matter (PM). Excess nitrogen in surface water leads to hypoxia (reduced oxygen in water) and harmful algal blooms.  More commonly in California, excess nitrogen is stored groundwater (used commonly as drinking water in the state) and is associated with human health problems such as adverse birth outcomes and blue baby syndrome (Methemoglobinemia).

In an effort to better understand nitrogen's pathways through the state, we have created a Mass Balance of nitrogen.  A mass balance tracks nitrogen flows in the state (inputs, outputs and storage) to develop a better understanding of how N affects ecosystem services and human well-being. 

Mass Balance Basics

A  mass balance is an efficient and scientifically rigorous method to track the flows of N in a system.  The underlying premise of a mass balance is that all of the reactive N entering (i.e., inputs) the study area must be exactly balanced by N leaving (i.e., outputs) and N retained in the study area (i.e., change in storage):

N Inputs = N Outputs + ΔStorage

A mass balance approach is very useful to compare the size of N flows and also to identify gaps in understanding the size and directions of these flows.  Some flows are difficult to quantify -- they are highly variable in time and/or space, or there are simply no methodologies to easily measure or predict the flows.  Nevertheless, knowledge of the relative magnitude of the flows is needed to make informed management and policy decisions for targeting N reductions. 

A significant component of the mass balance catalogs California's internal flows of nitrogen.  First, nitrogen is a highly volatile element and can quickly change forms.  Farmers who apply nitrogen to their fields as fertilizer know that nitrogen levels can quickly drop following an application, as the fertilizer is both consumed by plants and leaches into soil, quickly changing forms into nitrite and nitrate.  Too, we are a highly mobile people -- we transport food throughout the state and internationally; we move manure from livestock operations to fields for fertilization; we store, clean, transport, flush, and dispose of our water.  To understand where nitrogen enters and exits the systems, it is important to understand each stage along the way, and how nitrogen's presence affects our productivity and progress, but also our ecosystem and human health.  

 

Preliminary findings (see charts of all flows below): 

note on units png
  • Synthetic fertilizer is the largest statewide import (519 Gg N yr-1) of N in California

  • Manure production is the second largest N flow (416 Gg N yr-1) in California
  • Synthetically fixed N dominated the N flows to cropland
  • The biological N fixation that occurs on natural land (139 Gg N yr -1) has become completely overshadowed by the reactive N related to human activity in California
  • The synthetic fixation of chemicals for uses other than fertilizer is a moderate (71 Gg N yr-1) N flow
  • Urban land is accumulating N
  • Nitrogen exports to the ocean (39 Gg N yr-1 ) from California rivers accounts for less than 3% of statewide N imports
  • Direct wastewater export of N to the ocean (39 Gg N yr-1) is more than double the N in the discharge of all rivers in the state combined
  • Nitrous oxide (N2O) emissions are a moderate (38 Gg N yr-1) export pathway for N
  • Ammonia is not tracked as closely as other gaseous N emissions because it is not current regulated in the state
  • Atmospheric N deposition rates in parts of California are among the highest in the country, with the N deposited predominantly as dry deposition
  • The atmospheric N emitted as NOx or NH3 in California is largely exported via the atmosphere downwind (i.e. east) from California
  • Leaching from cropland (333 Gg N yr-1) was the predominant (88%) input of N to groundwater
  • The amount of evidence and level of agreement varies between N flows 

Statewide inputs

Inputs are measured as new sources of nitrogen into the state -- the creation of new reactive nitrogen through fixation and fossil fuel combustion, imported nitrogen (in water, fiber, feed and fertilizer) and atmospheric imports. 

statewide inputs pie chart

 Click to enlarge


Statewide outputs and storage

The fate of nitrogen in California is divided into nitrogen storage and outputs.  Ultimately, the only way for nitrogen to return to the atmosphere as inert N2 is through the process of denitrification (performed, much like the opposite process of fixation, by bacteria).  But as we see from the mass balance, only a small amount of nitrogen is denitrified; the rest either leaves state boundaries (food, in the case below, is food that is exported out of state, causing a loss of nitrogen within California, but not at its final destination), or is stored within the state, building on our existing pool of reactive nitrogen. 

The four major outputs and storage of nitrogen are nitrogen oxides, groundwater storage, N2, and ammonia.  NOx and NH3 (emissions largely from livestock manure)can both be re-deposited near their emission's source, but are considered outputs because they change forms before deposition (as particulate matter, or , for NOx, as acid rain.

The case for groundwater storage and N2 outputs is a complex component to the mass balance. In the chart below, 16% of our nitrogen is stored in California's groundwater. Unlike many states, were irrigation runs off into surface water, our irrigation run off leads primarily into groundwater. A total of 348Gg N enter groundwater from a variety of sources (leaching from cropland, natural land, manure and sewage). And by some estimates, that 348Gg of nitrogen, representing 22% of the state total N, stays in groundwater. But some research suggests that nitrate in groundwater can also be denitrified, returning it to inert N2 gas.

statewide outputs - No denitrification pie chart

If we include denitrification in our calculations, groundwater storage drops to 16% of the state total, and N2 increases from 7% to 13%. The range still places nitrate (NO3) storage in groundwater as one of the major nitrogen sinks in the state, but highlights the uncertainty around some chemical processes, particularly in difficult-to-monitor locations such as aquifers.

statewide outputs - with denitrification pie chart

 

Uncertainties and research priorities

The case of nitrogen in groundwater is an example of the California Nitrogen Assessment's work to catalog the level of certainty that scientists have about biological processes of nitrogen flows as well as management and policy practices that may improve the efficiency of nitrogen use in California.  

More information on the mass balance and California Nitrogen Assessment will be posted as it goes through scientific review, and will be open to public review in the future. Click here for more information on our review process.